GEOL 135
Lab 3 – Field Geochemistry Sampling
Fall 2010
Part 1: We will be investigating a drainage in centennial forest today, assessing the site and
splitting into 2 groups to draw up sampling plans for this site. We will use a conductivity meter
(this measures the electrical conductivity a water has, roughly a measure of the amount of
dissolved ions in the water) to initially investigate water chemistry changes in the area. The
question each group will be designing a sampling plan to address is:
Group 1 (Brie, Sam, Angel, Mike, Laura, James). High salt concentrations may be coming from
one of several sites, owned by different interests, one of which is the Sheraton Hotel who has
hired you to determine if they are responsible for any of this. How much of the salt is coming
from ‘your’ client’s site vs. other sites (like the university properties)?
Group 2 (Jordan, Emmy, Tim, Jacob, Nikki, Megan). Heavy metal contamination has been
discovered in a drainage pool this creek drains into, UVM has hired your group to determine:
what is the source of metal contamination into the stream and the processes contributing to metal
loading so a remediation strategy can be suggested?
Each group should come up with a sketch map and detailed sampling plan for this site given your
group’s question. The sampling plan should include the kinds of locations you would take
samples from, and what information you would want (for example, putting a piezometer in the
ballfield to draw groundwater to analyze for heavy metals). There is no necessarily ‘right’
answer to any of this, I am looking for your group to think through the problem and look at the
field site to design a good sampling protocol to address the questions posed.
Part 2: Some measurements cannot be done with samples back in the lab, they must be measured
in the field. Based on you conductivity survey, use the pH meters, Alkalinity Kits, and
colorimeters to measure key variables. Also utilize the syringes, filters, and tubes to gather water
and sediment samples as fitting to your sampling design.
pH:
pH is a measurement of the activity of the hydrogen ion in solution. A pH measurement system
consists of three parts: a pH measuring electrode, a reference electrode, and a high input
impedance meter. The pH electrode can be thought of as a battery, with a voltage that varies with
the pH of the measured solution. The pH measuring electrode is a hydrogen ion sensitive glass
bulb, with a millivolt output that varies with the changes in the relative hydrogen ion
concentration inside and outside of the bulb. The reference electrode output does not vary with
the activity of the hydrogen ion. The pH electrode has very high internal resistance, making the
voltage change with pH difficult to measure. The input impedance of the pH meter and leakage
resistances are therefore important factors. The pH meter is basically a high impedance amplifier
that accurately measures the minute electrode voltages and displays the results directly in pH
units on either an analog or digital display. Temperature compensation is contained within the
instrument, because pH electrodes and measurements are temperature sensitive. The temperature
compensation may be either manual or automatic. With manual compensation, a separate
temperature measurement is required, and the pH meter manual compensation control can be set
with the approximate temperature value. With automatic temperature compensation (ATC), the
signal from a separate temperature probe is fed into the pH meter, so that it can accurately
determine pH value of the sample at that temperature.
We are ALL going to calibrate a pH meter/electode using the automatic functions associated
with calibrating the electrode, AND record the mV readings corresponding to these buffers. To
calibrate, press the calibrate key on the meter, put the electrode in an appropriate buffer solution,
and wait for the measurement to stabilize – press enter when set and then rinse the electrode tip
and proceed to the next buffer (see individual meter instruction manuals for additional guidance).
Hit the calibrate button again when finished. We will use pH 10, 7, and 4 buffers for this
exercise. Switch modes and record the mV readings. You will record temperature and plot the
mV vs. pH readings and determine the slope (mV/pH unit) associated with each electrode.
Conductivity
We will use the accumet meter with the conductivity electrode to monitor conductivity, a
measurement of the electrical conductivity in a water. Pure water has a very low conductivity,
i.e., it does not transmit electrical charge very well. However, dissolving ions into water greatly
increases this conductivity. Conductivity electrodes are calibrated using a 1-point calibration –
immerse the probe into one of the two conductivity standards provided in purple-capped falcon
tubes. Allow the reading to stabilize, press calibrate and use the arrows to adjust the
measurement to the correct value. Now take a conductivity measurement of pure water from the
large bottle labeled 18M water and another reading of a sample from Saint Albans Bay.
Alkalinity
We will be using a Hach Co. digital titrator to do a basic titration for alkalinity and acid
neutralizing capacity (alkalinity measurements are for filtered samples, ANC is on unfiltered
samples). We wil use the 0.1600 N H2SO4 as the titrant, I’ll demonstrate loading the cartridge
and use of the titrator. Follow the instructions associated with the method to titrate for alkalinity
on a filtered SAB sample provided.
Colorimeter
We will use a portable spectrophotometer with pre-packaged reagent sets to measure a number of
chemical species colorimetrically (measure color intensity of specific colored complexes for
ions/molecules of interest). The Hach spectrophotometer will be used today to measure nitrate
and oxygen so you can each get used to running these kinds of analyses. When we are on the
Melosira, we will hopefully get through a number of samples for these species as well as
ammonia and nitrite. Instruction sheets are included for each of these measurements, use a
provided SAB surface water and determine the NO3- and O2 concentrations for that sample.